This invention relates to the palladium-catalyzed deprotection of allylic esters and ethers.
It is well known to use the allyloxycarbonyl group as a protecting group for a carboxylic acid, i.e., to esterify the carboxylic acid with allyl alcohol and to thereafter remove the allyloxycarbonyl group to convert the carboxylic acid group back to its original form after it has served its protecting function. For example, Ohtani et al in Journal of Organic Chemistry, 1984, Vol. 49, pps. 5271-5272, report that one crucial step in the synthesis of carbapenems is the final deprotection step of the C-3 ester function and cite as one example the cleavage of the allyl ester group by the action of palladium(0).
The following references all disclose cleavage of the allyloxycarbonyl function in carboxyl protected betalactam derivatives, such as penicillins, cephalosporins, and carbapenems, using potassium 2-ethylhexanoatein the presence of a catalytic amount of tetrakis(triphenylphosphine)-palladium(0) and triphenylphosphine:
Jeffrey et al, Journal of Organic Chemistry, 1982, Vol. 47, pps. 587-590; PA0 U.S. Pat. No. 4,314,942; PA0 U.K. Pat. appln. No. GB 2 128 187A, published Apr. 26, 1984, Example 21.
Kunz et al, Angew. Chem. Int. Ed. Engl., 1984, Vol. 23, pps. 71-72 report on the use of the allyl group as a removable carboxy-protecting group for the synthesis of labile O-glycopeptides. This article reports on the cleavage of the allyl ester moiety by reaction with about ten mole percent of tetrakis(triphenylphosphine)palladium(0) under argon in tetrahydrofuran and in the presence of a ten fold excess of morpholine as an acceptor nucleophile.
It is an object of this invention to provide a process for the deallylation of allyl esters and phenolic ethers which results in higher yields of the corresponding carboxylic acid or phenol then prior art processes.
It is a further object of this invention to provide such a process which can be conducted at lower temperatures and in shorter reaction times than prior art processes, thereby making possible its application to allyl esters having sensitive structural features that might be decomposed under reaction conditions involving higher temperatures and longer reaction times.